Preservation and diagenesis of hopanoids in recent lacustrine sediments of Priest Pot, England

Absolute concentrations of various bio- and geohopanoids have been determined in a 24 cm long core from anoxic Recent sediments of Priest Pot, a small highly productive lake in the English Lake District. Samples were Soxtherm extracted, derivatised by methods targeted at specific hopanoid groups, and quantified by GC-MS. Periodic acid/sodium borohydride treatment was used to cleave the side-chains of highly functionalised hopanoids, producing products more amenable to our detection methods. Biohopanoids are extremely abundant (totalling up to 3000 μg/g dry sediment), particularly those with composite or highly (penta- and hexa-) functionalised side-chains, suggesting that these are important precursors to sedimentary hopanoids in Priest Pot. Downcore biohopanoid abundance profiles show a general diagenetic decline with a marked dip at 5–8 cm, which we interpret as a change in bacterial input. Since this is most marked in the composite hopanoids rather than bacteriohopanetetrol, we suggest that these two hopanoid groups have at least partly different sources, and may thus have potential as environmental markers. The geohopanoids are also abundant (totalling up to 1700 μg/g dry sediment) being dominated by the ββ and αβ bishomohopanoic acids, ββ and αβ bishomohopanol and hop-17(21)-ene. Downcore plots show no apparent conversion of free bio- to geohopanoids with depth, which suggests that incorporation into kerogen may be commencing at an early diagenetic stage in the sediments.